Command + Option + R on Mac (Control + Alt + R on Win) will reset the sliders for the Local Adjustment Tools in Adobe Camera Raw. This shortcut will work regardless of whether you have a local correction selected or not. (If you don’t have one selected, it’ll simply reset the sliders to zero for the “next” correction that you create. Note that in ACR, attempting to create an “empty” correction will result in a warning dialog.)
You can also reset the sliders by right-clicking the pin in the preview area and choosing “Reset Local Correction Settings” from the popup menu, or choose the same entry from the flyout menu in the local correction pane.